1. Field of the Invention
The present invention relates to plasma systems capable of thermal spraying of powdered materials for coating on a workpiece.
2. History of the Prior Art
It is known to provide a plasma system in which powders of metal or other composition are delivered to a plasma gun for introduction into a plasma stream or flame produced by the plasma gun. The plasma stream, which is created by a flow of inert gas with the application of an electrical power source and typically in the presence of low pressure, provided by a vacuum source, is directed from the plasma gun onto a workpiece or other target where the powder is deposited to form a coating. The powder, which may be preheated before introduction into the plasma stream at the gun, melts as it is entrained into and carried by the plasma stream so that a relatively dense coating is formed on the workpiece.
An example of such a plasma system is provided by U.S. Pat. No. 4,328,257 of Muehlberger et al., which patent issued May 4, 1982 and is commonly assigned with the present application. In the plasma system described in the Muehlberger et al. patent, a low pressure source in the form of vacuum pumps is coupled to an enclosure containing a plasma gun and a workpiece to direct the plasma stream from the plasma gun to the workpiece at supersonic speeds. A powder feed mechanism heats and delivers powder into the side of the plasma gun for introduction into the plasma stream.
In plasma systems of the type described in the '257 patent of Muehlberger et al., the plasma gun which has an anode and a cathode is water cooled by supplying cooling water from a water booster pump to a water inlet. From the water inlet, the cooling water circulates through a predetermined path within the plasma gun before exiting via a water outlet for return to the water booster pump. Cooling water from the water booster pump is provided to the water inlet of the plasma gun by a hose having a conductive inner tube which functions as the cathode connection to the plasma gun and which is coupled to a plasma power supply. The water outlet of the plasma gun is coupled by a second hose to return the water to the water booster pump. The second hose has a conductive inner tube which functions as the anode connection to the plasma gun and which is coupled to the plasma power supply. A tube coupled to a powder feed mechanism provides powder to the plasma gun with the help of a carrier gas flowing under pressure. Another tube couples a source of inert plasma gas to the plasma gun to provide plasma gas to the gun.
In most applications of plasma systems of the type described in the '257 patent of Muehlberger et al., the plasma gun and the connecting portions of the water-supplying electrode hoses and the powder and plasma gas supply tubes are subjected to moderate temperatures which are not substantially in excess of about 500.degree. F. This does not adversely affect the hoses, which are typically Teflon-coated on the exterior thereof. Nor do such temperatures adversely affect the powder and gas supply tubes.
However, for certain applications of the plasma system, the plasma gun and the connections thereto may be subjected to temperatures substantially in excess of 500.degree. F. This may occur, for example, where the plasma gun is located at the interior of a circular workpiece in order to spray the inner surface thereof. As the circular workpiece undergoes rotational motion relative to the plasma gun for spraying of the interior surface thereof, the temperatures in the vicinity of the plasma gun may be as high as 2,000.degree. F. Temperatures of this magnitude do not adversely affect the plasma gun, which is water-cooled and essentially of metal construction. However, such high temperatures adversely affect the connecting hoses as well as the powder and plasma gas supply tubes. The Teflon-coated hoses rapidly deteriorate in the presence of such temperatures. In addition, conventional non-cooled powder and gas supply tubes will not function properly at temperatures of this magnitude.
It is therefore an object of the present invention to provide an improved plasma gun assembly. A more specific object of the invention is the provision of a plasma gun assembly capable of withstanding the high temperatures produced during certain operations such as spraying the interior of a circular part.